1. Technical Field
The present disclosure relates to the field of liquid crystal displaying, and more particularly, to a light emitting diode (LED) drive circuit.
2. Description of Related Art
In conventional liquid crystal displays (LCDs), alternating current (AC) power must be transformed into direct current (DC) power for driving LEDs. A prior art LED drive circuit is as shown in FIG. 1. The LED drive circuit comprises a square wave generator 100, an LLC resonance circuit 200, a rectifying circuit 300, a boost circuit 400 and an LED unit 500. The square wave generator 100 may be a half-bridge inverter or a full-bridge inverter. Taking a half-bridge inverter as an example, the half-bridge inverter comprises a half-bridge drive integrated circuit (IC) 10, and two switches S1, S2 (diodes or field effect transistors (FETs)) connected in series. The two switches S1, S2 are turned on or off alternately, and drive signals thereof are complementary signals having a certain duty ratio. An LLC resonance circuit 200 is formed by a serial resonance inductor Ls, a serial resonance capacitor Cs and a transformer. Because the LLC resonance circuit 200 is connected at a middle point of the half-bridge, the serial resonance capacitor Cs also serves the function of blocking the DC current. The transformer comprises an inductor Lm acting as a primary winding and inductors L1 and L2 acting as secondary windings. At the secondary side of the transformer, a rectifying circuit 300 having a center tap is formed by rectifying diodes D11 and D26. A square-wave voltage outputted by the square wave generator 100 is modulated by the LLC resonance circuit 200 and the rectifying circuit 300 into a square-wave voltage of a reduced level. The reduced square-wave voltage signal is then boosted by the boost circuit 400 into a voltage necessary for the LED unit 500. The boost circuit 400 comprises an inductor L3, a diode D12 and a switch S3 (e.g., an FET). The diode D12 has an input terminal connected to the inductor L3 and an output terminal connected to the LED unit 500. The switch S3 has a terminal connected to a middle point between the inductor L3 and the diode D12 and a terminal connected to the ground. The switch S3 is controlled by a pulse voltage (i.e., PWM) to boost the DC signal outputted by the rectifying circuit 300 into a voltage for powering the LED unit 500. As the LED drive circuit shown in FIG. 1 comprises the square wave generator 100, the LLC resonance circuit 200, the rectifying circuit 200, the boost circuit 400 and the LED unit 500 at the same time, the cost is high and the efficiency is low.